text: Green roofs can reduce stormwater runoff via water-wise gardening techniques. Green roofs play a significant role in retrofitting the Low Impact Development (LID) practices in urban areas. A study presented at the Green Roofs for Healthy Cities Conference in June 2004, cited by the EPA, found water runoff was reduced by over 75% during rainstorms. Water is stored by the roof's substrate and then taken up by the plants, from which it is returned to the atmosphere through transpiration and evaporation.
  Green roofs decrease the total amount of runoff and slow the rate of runoff from the roof.  It has been found that they can retain up to 75% of rainwater, gradually releasing it back into the atmosphere via condensation and transpiration, while retaining pollutants in their soil. Many green roofs are installed to comply with local regulations and government fees, often regarding stormwater runoff management. In areas with combined sewer-stormwater systems, heavy storms can overload the wastewater system and cause it to flood, dumping raw sewage into the local waterways. Often, phosphorus and nitrogen are in this category of environmentally harmful substances even though they are stimulating to the growth of plant life and agriculture.  When these substances are added to a system, it can create mass biological activity since they are considered limiting factors of plant growth and by adding more of them to a system, it allows for more plant growth.